1. Field of the Invention
The present invention relates to a decoder receiving broadcasting which uses picture compression technology, such as MPEG (Moving Picture Experts Group), and converting the data into raster data inputted to a CRT (Cathode Ray Tube) to display pictures on the CRT.
2. Description of the Background Art
Industries and countries have been using different frequencies for video signals so far. For example, the cinema industry has been mainly using a frame frequency of 24 Hz with projectors and the television industry has been using different frame frequencies (60 Hz, 59.94 Hz, 50 Hz) to display television pictures in various countries. While the picture compression technology such as MPEG is increasingly applied to broadcastings nowadays, the standards for the picture compression of MPEG relate to transmission carried on packet data, which do not uniquely define picture formats such as the frequency of transmission data, the frame size, the scanning line system, etc. because of the above-mentioned historical background.
For instance, ATV (Advanced Television) provides picture sources having various standards as shown in Table 1. The standards shown in Table 1 provide various kinds of combinations of the picture rate, the number of picture elements in the horizontal direction, the number of lines in the vertical direction, and the scanning system.
TABLE 1 ______________________________________ picture rate (Hz) frame size scanning system ______________________________________ 60.00 1920 .times. 1080 interlace 59.94 1920 .times. 1080 progressive 30.00 1280 .times. 720 progressive 29.97 640 .times. 480 progressive 24.00 704 .times. 480 progressive 23.96 640 .times. 480 interlace 704 .times. 480 interlace ______________________________________
The receiver in the ATV system must support all of the formats shown in Table 1. However, when a picture is displayed on the CRT of television by using such a low frame frequency as is used by a projector, a visual sensation called flicker is usually produced and the displayed picture is deteriorated.
Accordingly, it is necessary to convert the picture rate to a higher frame frequency at output to the CRT. FIG. 14 shows the concept of the relation between the decoder and the CRT. As shown in FIG. 14, the decoder 100 converts packet data 102 inputted to the decoder 100 into raster data 103 and outputs it to the CRT 101. The raster data 103 has a data format related to the scanning lines in the CRT 101.
Suppose that the picture rate of the video signal sent on the packet data 102 is 24 Hz and the frame frequency for display in the screen is 60 Hz. While the packet data 102 is sent to the decoder 100 at the rate of 24 pictures per second, the raster data 103 is outputted from the decoder 100 at the rate of 60 frames per second. Then the decoder 100 has to generate the raster data 103 while adjusting the difference between the number of input pictures and the number of output frames.
FIG. 15 provides a timing chart showing the relation between the packet data having the picture rate of 24 Hz and a processing time of 30 Hz and the raster data having the frame frequency of 60 Hz. It is assumed that the interlaced scanning is used.
At time t0, the packet data 110 related to a picture A arrives at the decoder 100. Generally, the packet data includes data about the picture format of the picture A in its header. Hence, the decoder 100 can generate the raster data from the received packet data 110 according to the picture format. At time t1, although the packet data 110 about the picture A has not been completely processed, the operation of outputting the raster data 111 on the even scanning line side of the picture A, or in the top field, is started because the necessary information has already been processed. At time t2, the processing of the packet data 110 is finished and the operation of outputting the raster data 112 on the odd scanning line side in the picture A, or in the bottom field, is started. Between time t2 and t3, the processing of the packet data 113 about a picture B is started. At time t3, the operation of outputting the raster data 114 on the even scanning line side in the picture A is started. The raster data 115 and 116 corresponding to the packet data 113 are read between time t4 and t6.
FIG. 15 shows the packet data 110, 113, and so on having a large amount of data which are adapted to HDTV (High Definition Television). Since the data are adapted to HDTV, they require a relatively long time for processing, e.g., 1/30 of a second. In contrast, in the case of packet data for common television with a smaller amount of data, 1/60 second, for example, is sufficient to process the data.
Generally, the decoder is required to conduct the above-mentioned conversion. When the decoder handles packet data and raster data each limited to one type, the standards such as the picture rate and the amount of data of the packet data and the frame frequency, the frame size and the scanning system of the raster data are uniquely determined. Accordingly, a simple structure can be used as the decoder even if it is used to receive broadcasting requiring synchronous processing.
However, the ATV standard requires processing of data of so many kinds of formats that the circuit structure becomes complicated when circuits adapted to the large number of kinds are separately produced and combined.
FIG. 16 is a timing chart showing, in time, the approximate amounts of data of the individual video formats after MPEG decoding, or the amounts of data (time required for processing), and the differences among processing timings of the packet data with different picture rates in the decoder. Table 2 shows the amounts of data and the picture rates of the data sequences 120-127 in FIG. 16. The processing cycles of the data sequences 124 and 127 are for the picture rate of 24 Hz, which provide an alternate cycle of 30 Hz and 20 Hz in which the timing is shifted in even and odd places in the order of processing. As shown in Table 2, the picture format can be classified into eight kinds on the basis of the combinations of the amount of data and the picture rate (processing cycle).
TABLE 2 ______________________________________ data sequence amount of data (sec.) picture rate (Hz) ______________________________________ 120 1/60 60 121 1/120 60 122 1/60 30 123 1/60 24 124 1/60 30/20 125 1/30 30 126 1/30 24 127 1/30 30/20 ______________________________________
When decoding compressed data into macro-block data, a higher processing capability is required than when handling un-compressed data. If the decoder does not have enough processing capability, correct video signals may not be generated due to lack of information necessary for decoding. If a display is made in the CRT with incorrect signals, the disordered display on the CRT will cause the viewer to mistake it for a trouble of the receiver, or the CRT may actually be broken. Hence, the configuration must be made to continuously output correct video signals.
Furthermore, if the supply of the packet data to the decoder is interrupted, the receive timing of newly supplied packet data including information about a new picture does not always coincide with the receive timing of the data before interruption.
FIG. 17 provides a timing chart showing the relation between the packet data received at the decoder and the raster data correspondingly outputted. It is assumed that the decoder 100 shown in FIG. 14, for example, receives the packet data 130-133 in a normal state. However, the packet data 131-133 may not be received for some reason, e.g., a lack of processing capability of the decoder, interruption of the broadcasting, too weak wave, a change of channel, etc. Then the decoder 100 must continuously output the raster data 138-142, in addition to the raster data 135-137 normally generated on the basis of the packet data 130. Otherwise the picture will be disturbed or become black, which is not suitable as a civil requirement television monitor.
The packet data 134 which comes first after the operation is restarted may be received at a shifted timing. In order to process the data at the same timing as the packet data 130, the data must be received at the timing of the packet data 133 shown by the dotted line. Accordingly, it is necessary when the packet data 134 is received to re-establish the synchronization of the operating timing in the decoder 100 and the synchronizing signal of the CRT 101.